Successful detection and isolation of fetal cells from maternal blood would permit non-invasive and risk-free monitoring of fetal well-being and prenatal diagnosis of birth defects. Such cells could be obtained as early as the first 8-10 weeks of pregnancy and, unlike chorion villus and amniocentesis assays, represents no risk to either mother or fetus. This would permit the use of a test based on this research on all women-regardless of age or other risk factors. It could also serve as a rapid pre-screening test which could change the risk factors for other more invasive tests. A better understanding of the nature of this transplacental traffic of fetal cells and the maternal immune response to these cells may also aid in the understanding of closely related problems of spontaneous abortions and infertility. In this grant we propose use of special and unique high-speed (100,000 cells per second) multiparameter flow cytometric analysis and sorting techniques for identification and isolation of rare (.02 - .001 percent) fetal cells directly from maternal blood on the basis of monoclonal antibodies against paternal HLA antigens, against fetal trophoblastic antigens, or against selected proto- oncogene products elevated in fetal cells. Isolated cells will be examined genetically by use of chromosome specific c-DNA probes and either fluorescence in-situ hybridization or dot-blot autoradiography of whole cells to confirm fetal cells (Y-chromosome specific probes) or number of copies of specific chromosomes (chromosome 21-specific probes). Maternal recognition and action against fetal cells will also be examined by in-vitro mixed leukocyte reactions between cord and maternal blood cells. Down- regulation and/or masking of fetal paternal HLA antigens and their increased expression by induction with interferon and other agents will also be explored using cord blood cells. The utility of fetal nucleated erythrocytes, present in large numbers during pregnancy, for prenatal diagnosis will also be examined.